The present invention relates to power conversion equipment which has a frequency control device and a power control device and also has a switch device for switching between the two control devices.
FIG. 1 shows a block diagram of a system configuration of conventional power conversion equipment. Referring to FIG. 1, DC power transmission equipment has a DC system 10 connected between an AC system 1 at the sending terminal and another AC system 1' at the receiving terminal. A transformer 2 is connected to the AC system 1, and a rectifier 3 is connected to the transformer 2. A DC power from the rectifier 3 is supplied to one end of a DC transmission line 5 through a DC reactor 4. The other end of the DC transmission line 5 is connected to an inverter 3' through another DC reactor 4'. The inverter 3' converts the input DC power to an AC power.
A potential transformer 6 and a current transformer 7 are coupled between the DC reactor 4 and the DC transmission line 5. The voltage and current detected at the transformers 6 and 7 are supplied to a power detector 8. The detected power thus obtained is supplied to the control input end of a power control device 9. Preset power reference is also supplied from a power setter 9a to the power control device 9. Based on the detected power data and the preset power reference thus received, the power control device 9 produces a current reference. The current reference is supplied to a control device 11 at the sending terminal as well as to a control device 11' at the receiving terminal through a communication line 12. Each of the control devices 11 and 11' has a current control circuit, a voltage control circuit, an extinction angle control circuit and the like. Output signals from the control devices 11 and 11' are used for gate control of SCR's constituting the rectifier 3 and the inverter 3' to control the transmission power.
When the power conversion equipment as shown in FIG. 1 is operated in the frequency control mode, a frequency control device 13 as shown in FIG. 2 is used in place of the power control device 9. The frequency control device 13 receives a preset frequency signal from a frequency setter 14 as well as a detected frequency signal of the AC system 1 through a potential transformer 15 and a frequency detector 16. The frequency preset by the frequency setter 14 is fixed at, for example, 50 Hz, and a signal indicating the difference between the preset frequency and the detected frequency is obtained from the frequency control device 13. The difference signal is supplied to the control devices 11 and 11' shown in FIG. 1 through a current conversion circuit 17, so that the power frequency remains constant.
The power conversion equipment is generally operated in either the power control or frequency control mode. However, depending on the circumstances, both the power control device and the frequency control device may be installed and they may be selectively used as needed. Assume that power conversion equipment having a power control device and a frequency control device is operated in the frequency control mode and is transmitting power of 100 MW. Assume also that a switching instruction to the power control mode is produced from a remote control center or operating console, and that the preset power of the power setter 9a is 500 MW. Referring to FIG. 1, if the AC system 1 has a sufficient short-circuit capacity with reference to the DC conversion installation capacity, that is, if the AC system 1 is a strong AC system, the problems such as frequency variation may not be caused even if the transmitted power abruptly increases from 100 to 500 MW. However, in the case of a weak AC system 1 wherein the capacity ratio of the AC system 1 to the DC system 10 is below about 3, significant frequency variation will occur to cause a system failure. In order to prevent such problems, preset power from the power setter 9a must be changed to a value compatible with the current system state before the operation mode is changed from the frequency control mode to the power control mode. If a mode changeover device is constructed to be operated manually, a serious fault may occur by an erroneous manual operation. Furthermore, the manual operation does not meet a requirement of rapid changeover of the operation mode. Thus the mode changing should be done automatically.